This invention relates to sealed rechargeable lead-acid cells and batteries of the starved electrolyte type in which oxygen generated toward the end of charge and on overcharge is internally recombined within the battery at high efficiencies. More particularly, the invention relates to such a lead-acid cell that has a high rate discharge capability.
A prominent failure mode for lead-acid batteries, whether of the open or sealed type, is corrosion of the positive current collector grid. Thus, it has been a common practice in the industry to insure that the positive grid has adequate thickness to withstand the degradation caused by corrosion during service. For the same reason, oftentimes the positive grids/plates are made considerably thicker than the opposite polarity negative grid/plates used in the battery.
To achieve optimum high rate discharge capability, in theory one would prefer to use thinner plates to reduce the current density on discharge. However, corrosion, particularly of the positive grid as aforementioned, has placed limitations on how thin the plates can be made in practice. Use of thin grid/plates also presents problems in assembling of the cells. U.S. Pat. No. 3,862,861 to McClelland et al discloses a normally sealed rechargeable lead-acid cell of the starved electrolyte type in which oxygen generated toward the end of charge and during overcharge has free access to the negative plate active material through voids in the plates/separator porous matrix. In practice, plates made in accordance with the McClelland et al patent have typically had a thickness of about 0.044 inches (grid typically 0.032 inches), and the patent discloses a grid thickness ranging from 0.020 inches to 0.45 inches. It is recognized in that patent that the grid strands in the positive plate are slowly converted from lead to lead dioxide, and too thin a strand thickness will shorten the life of the cell.
European patent application No. 0 141 568 A1 to Okada et al discusses grid/plate thickness for starved electrolyte batteries and concludes that a battery to have superior high rate discharge characteristics without sacrificing various other characteristics should optimally have a grid thickness for the positive plate of 3 mm to 4 mm, whereas the aforementioned grid thickness of the commercial embodiment of U.S. Pat. No. 3,862,861 was approximately 1.0 mm (0.039 inches) or less.
It is also known that in sealed lead-acid recombinant batteries the possibility of short-circuiting increases with decreasing inter-plate spacing. Positive plate growth during cycling adds to the problem. European patent application No. 0 141 568 teaches an inter-plate spacing of 0.95 mm (0.037 inches), and that the thickness of the separators are desirably in the range of 0.4 to 0.25 times the thickness of the positive plates; i.e., suggesting a minimum inter-plate spacing of about 0.75 mm (0.030 inches).
U.S. Pat. Nos. 3,395,043 and 3,494,800 to Shoeld disclose flooded lead-acid batteries (nonsealed) employing lead foil substrates of 0.002 inches thick on which is applied on both surfaces active material to a thickness of 0.006 inches. A separator consisting of three layers of conventional rubber latex impregnated kraft paper of 0.008 inches thickness per layer is interposed between the electrodes and wound into a spiral configuration. To Applicant's knowledge, this battery was never commercialized and its technical practicality has to be doubted.
It is an object of the subject invention to produce a rechargeable lead-acid cell of the sealed recombinant type having extremely high discharge rate capability, provided in part by using ultrathin grid/plates, close inter-plate spacing and a construction in which positive grid corrosion is minimized to the extent that it is no longer a significant problem, and wherein an extremely high plate surface area is defined per unit of volume of cell element, thereby enhancing the high rate capability of the cell.